The present invention relates to improved extrusion and injection moldable ceramic compositions. More particularly, the present invention is directed to novel extrusion and injection moldable ceramic compositions containing ceramic or metal powder and a polyacetal binder.
Ceramic materials are of critical importance for a number of high temperature, high performance applications. Recently, there has been substantial interest in the development of ceramic compositions for critical engine parts including reciprocating engines, gas turbine and rocket engines. These applications require a unique combination of properties such as high specific strength, high temperature mechanical property retention, low thermal and electrical conductivity, hardness and wear resistance, and chemical inertness. However, the inability to produce complex shapes of high dimensional accuracy and sufficient strength using an economical fabrication technique has prevented ceramic materials from fulfilling their potential in these critical high temperature, high performance applications.
Several processes have been used in an attempt to form ceramic bodies. Among such processes include pressing ceramic powder into a greenbody followed by sintering or by hot pressing and subsequently shaping or machining the sintered body to produce the finished product. Another technique is slip casting in which the ceramic particles are dispersed in water, the slurry placed in a mold and the water removed to form a greenbody. The pressing techniques have been found unsuitable to form ceramic articles of complex shapes and which must meet specific design specifications. The slip casting technique is time consuming and has not yielded greenbodies of sufficient strength.
In view of the problems associated with the prior techniques, injection molding has been increasingly used to form ceramic articles. Injection molding is a process wherein a moldable composition is forced into a mold or die. The injection molding process facilitates a rapid and repeated forming of a plurality of articles having a consistency with close dimensional tolerance. The injection molding process also minimizes the amount of shaping or machining that may be required to produce a finished article.
The injection molding process typically involves forming a ceramic greenbody by injection molding a composition comprising ceramic powder dispersed within a thermoplastic polymer, burning out the polymer, and sintering the resulting porous greenbody to a dense ceramic part with the same shape. The thermoplastic binder acts as a fluidizing agent to distribute the injection pressure throughout the mold and as the material which holds the ceramic particles in the shape of the mold after the part is ejected. A typical ceramic powder/thermoplastic polymer composite has a very high content of the ceramic particles, typically from about 50 to about 87 volume % and a minimum of the binder material to hold the particles together in desired shape. A useful binder material for ceramic injection molding is a polyacetal resin as disclosed in U.S. Pat. No. 4,624,812, the entire contents of which are herein incorporated by reference.
A typical injection moldable ceramic composition will also contain a minor binder component which is often a thermoplastic, wax or oil, plasticizers which increase the fluidity of the ceramic-binder mixture, and processing aids such as surfactants which improve the wetting characteristics between the plastic binder and ceramic during mixing to form the composite.
A summary of injection molding applied to the fabrication of molded ceramic bodies is provided in an article entitled "Review: Fabrication of Engineering Ceramics by Injection Molding. I. Materials Selection", M. J. Edirisinghe et al, International Journal of High Technology Ceramics, Vol. II, 1986, pp. 1-31.
One of the advantages of using a polyacetal binder in an injection moldable ceramic composition is its relatively easy ability to undergo thermal degradation during the binder removal process. Thus, it has been found that the time for binder removal can be drastically reduced when a polyacetal resin is used as the binder. However, the property which renders the polyacetal binder to readily undergo thermal degradation also renders it difficult to produce defect-free parts when the binder removal process is shortened. For example, it has been found that the relatively easy ability of the polyacetal resin to degrade to low molecular weight fragments upon heating can be disadvantageous during the steps of compounding the ceramic powders with the polyacetal binder. During compounding, the ceramic powders generate a high shear, which can cause a premature degradation of the polyacetal binder. The ceramic green bodies thus formed may contain internal voids or cracks due to premature off-gassing and be of insufficient strength to maintain full integrity during handling subsequent to the molding operation. The premature degradation of the polyacetal binder, thus, may ultimately reduce the density and strength of the ceramic article.
Another difficulty found in processes for molding ceramic compositions is yielding a uniform mix of the sinterable powder within the binder in view of the high level of ceramic or metal power which is present in the composition. Thus, the high torque needed for compounding the sinterable powder with the binder can push the typical compounding apparatus beyond its functional limits. Besides damaging equipment, often powder agglomerates or dry areas devoid of binder are present in the composition. Difficulties in filling the mold and consequent deformed parts or internal cracks or voids are the result. While it is known, in general, to add wetting agents or surfactants to the injection moldable ceramic composition, there is still a need in this art to find particular dispersing aids which will uniformly disperse the ceramic within the binder and which can aid in reducing the torque and viscosity of the composition to allow homogeneous mixing of the ceramic and binder components.
Accordingly, it is an object of the present invention to provide an extrusion or injection moldable ceramic composition containing a polyacetal binder which has improved polyacetal stability during the compounding of the ceramic powder with the binder.
It is a further object of the present invention to provide a moldable ceramic composition containing a polyacetal binder which has improved dispersion of the ceramic powder within the polyacetal binder during compounding.
It is still a further object of the present invention to provide a moldable ceramic composition containing a polyacetal binder which has both improved stability of the polyacetal binder during compounding and improved dispersion of the ceramic powder within the polyacetal binder.
Yet another object of the present invention is to provide a process for producing ceramic articles by extrusion or injection molding ceramic compositions containing a ceramic or metallic powder dispersed within a polyacetal binder into ceramic green bodies, treating the green bodies such as by heating or contacting with solvent to remove the binder and then sintering or reaction bonding the green body to yield defect-free and carbon-free ceramic parts.